Overwhelmed... Is my calculation/my knowledge correct!?

I have already addressed your "accusations" above.

But I am surprised at how much some people sugarcoat their children through my eyes. Does a teenager really need a couch and table in addition to their bed? Does it have to be a 15sqm room to virtually set up an apartment there? Why is 4sqm of play area no longer enough for playing with Lego nowadays? Our teenager moves around within 8sqm in his current 20sqm room. He doesn’t use the rest at all. He is actually looking forward to his smaller room. "Then I don’t have to walk so far anymore." And the youngest zooms through the living room and hallway on his balance bike, chases the cats, or plays in his play corner (1x1.5m carpet in the living room).

For the OP: from the discussion, which threatens to miss your topic, you can take away that you should carefully consider what you need. If you don’t have children yet, it’s a bit hard to estimate that. I always had the idea that the kids play in the daycare center. Well, now it’s mostly the living room. Define clearly for yourselves what is important to you. Maybe think not only about the children but also about yourselves. YOU will still live there when the children have long since moved out. YOU will still be paying for it.

So now I’m sending you the list first.

I wonder how little time most people spend in the bedroom. For me, a large bedroom is important. I iron the laundry there and like to watch a movie in bed in the evening; for that, 12 sqm would be far too small.

Best regards
Sabine

Evolith, I have expressed myself here for the first time...

Exactly, I primarily live there, which is why, for example, I planned our bedroom in the north. A third room faces south. We use that (since we have no children) as a hobby/workroom, and I use it during the day; sleeping at night, I prefer it cooler in the room and don’t need sun or much more space than just enough for our bed to fit well (and, if you don’t have a dressing room, enough space for a wardrobe).

Especially when I consider that the children won’t live in the house forever, I would think carefully about what I can do with these rooms FOR MYSELF then. For many couples, it happens that they eventually sleep separately (because one starts snoring terribly). A good solution is when you can put double beds in both rooms (since you don’t want to always sleep separately and want to have all options), but I don’t know how that would work in 10 sqm. I can also use the former children’s room as a guest room; it also makes sense if you can at least put a smaller double bed there (the children might visit again and possibly as a couple). I can use one room as a hobby/workroom and then, see above, I find such a room in the brighter location facing the garden more useful, since it is used during the day, unlike the bedroom.

Especially when considering how to use children’s rooms after the child-rearing phase, such a planning is out of the question.

I have space for ironing in the utility room, and if I'm lying in bed watching a movie, I don't need any more space??? (not that we don't do that sometimes, but the TV will probably be mounted on the wall)

Oh dear, the poor cats. Hopefully, the shouting won't be too bad when they start using their claws...
It's at least funny that a young person finds the walking distances on 20m² too far ;-D
For example, I've never experienced a shared flat where the smallest room was fought over *lol*
But in the end, everyone plans for themselves, you are of course right about that...